function seomix_seo_img_add_alt_tags($content){
global $post;
preg_match_all('/<img (.*?)\/>/', $content, $images);
if(!is_null($images)) {
foreach($images[1] as $index => $value) {
if(!preg_match('/alt=/', $value)){
$new_img = str_replace('<img', '<img alt="'.get_the_title().'"',
$images[0][$index]);
$content = str_replace($images[0][$index], $new_img, $content);}}}
return $content;}
add_filter('the_content', 'seomix_seo_img_add_alt_tags', 99999);